COMPARISON OF THE EFFECTIVENESS OF ELECTRIC IR AND OTHER ENERGY-SOURCES TO CURE POWDER COATINGS

A study has been conducted to compare the effectiveness of electric IR and other energy sources to cure powder coatings. Four energy sources (high-intensity electric IR, gas IR, electric convection oven, and ga s convection oven) have been employed to cure nine commercially availa ble powders. The powders include: white epoxy, black epoxy, white acry lic, white urethane, white polyester, white polyester hybrid, white ny lon, clear polyester, and fine white polyester (classified by manufact urer to less than 25-30 mu). Manufacturer's curing recommendations (ti me-at-peak metal temperature) were used for convection oven curing. Po wder cure in electric and gas IR ovens was optimized using experimenta l methods. The curing conditions are presented as time-at-voltage (ele ctric IR) and time-at-BTU expressed as water column pressure (gas IR). Physical and performance properties of the nine coatings cured with t he four different energy sources are presented. Films of each powder w ere deliberately under- and over-cured and evaluated to note changes i n physical and performance properties with the degree of cure. These d ata demonstrate the efficiency of using IR curing methods to obtain op timum performance properties in powder coatings. The mechanisms by whi ch film formation and cure occur in powder coating materials depends o n the specific means used to impart heat to the coating and the coated substrate. All of these methods of cure involve conduction, convectio n, and thermal radiation processes. The fundamental concepts of each o f these mechanisms of heat transfer are reviewed. Special attention is given to convective and thermal radiation (IR) processes, as these tw o methods of heat transfer are of primary importance to the methods of powder cure evaluated in this work. Discussion is presented which def ines the variables important to each process, and the effects of these variables on cure in powder coatings. Possible mechanisms for the cur e process of powder coatings applied to substrate materials are presen ted for each of these processes, and the implications of these process es are discussed. In the case of IR cure of powder coatings, some ques tions are raised regarding the exact mechanism of the cure process. Th e advantages and disadvantages of convection and IR cure of powder coa tings are addressed in detail. The powders evaluated were examined wit h photoacoustic FT-IR (PA FT-IR) spectroscopy and differential scannin g calorimetry (DSC). The DSC work ascertained the melting and curing t emperatures of the powder coatings. Most of the coatings melted in the 160-180-degrees-C range and cured in the 220-240-degrees-C range. The PA FT-IR work illustrated the transparency of the coatings to IR in t he 1-3 mum range and the infrared absorption in the 3-20 mum range. Th ese infrared data also raised questions concerning the cure mechanism with respect to substrate conduction or coating absorption of the infr ared radiation. Evidence collected during the course of this work sugg ests that the coatings cure through conduction with some absorption of IR by the coating. The conduction of the energy, primarily from the s ubstrate to the coating, allows the coating from the substrate outward s to the coating surface. A model has been developed for describing ho w powder particles fuse and coalesce into a continuous film structure. Parameters, such as density of the powder coating, solubility paramet ers, surface tension, and melt viscosity, were correlated with the abi lity of the powder particles to flow together and form a continuous fi lm structure.